1. Field of the Invention
The present invention relates to a rotary compressor which compresses a refrigerant by a rotary compression element to discharge it, a method for manufacturing the same, and a defroster for a refrigerant circuit using the same.
2. Description of the Related Art
Conventionally, in a multi-stage compression type rotary compressor, a refrigerant gas-is sucked through a suction port of a first rotary compression element into a low-pressure chamber side of a cylinder, compressed by the operations of a roller-and a vane to have a medium pressure, and discharged into a, sealed vessel through a discharge port of the side of a high pressure chamber of the cylinder. Then, the refrigerant gas having the medium pressure in the sealed vessel is sucked through a suction port of a second rotary compression element into the low-pressure chamber side of the cylinder, undergoes second-stage compression through the operations of the roller and the vane to have a high temperature and a high pressure, and is discharged from the discharge port of the high-pressure chamber side. The refrigerant thus discharged from this compressor flows into a radiator to radiate its heat, is squeezed by an expansion valve to absorb heat at an evaporator, and sucked into the first rotary compression element, which cycle is repeated.
In such a multi-stage compression type rotary compressor, especially when, for example, carbon dioxide (CO2) having a large difference between the high and low pressures is used as the refrigerant, as shown in FIG. 5, a pressure of the discharged refrigerant reaches 12 MPaG in the second rotary compression element where the refrigerant has the high pressure (HP) and becomes 8 MPaG (medium pressure: MP) in the first rotary compression element which is the lower-stage side (where a suction pressure LP of the first rotary compression element is 4 MPaG). As a result, a differential pressure at the second stage (difference between the suction pressure MP of the second rotary compression element and the discharge pressure HP of the second rotary compression element) becomes a large value of 4 MPaG. Especially when an outside air temperature is low, the discharge pressure MP of the first rotary compression element becomes lower and, therefore, the second-stage differential pressure (difference between the suction pressure MP of the second rotary compression element and the discharge pressure HP of the second rotary compression element) increases further, so that a compression load of the second rotary compression element increases to bring about a problem that durability and reliability deteriorate.
Therefore, conventionally, by altering a dimension of thickness (or height) of the cylinder of the first rotary compression element so that a displacement volume of the second rotary compression element may be smaller than that of the first rotary compression element, a displacement volume ratio has been set so as to reduce a differential pressure at a second stage.
By such a setting method, however, the thickness (or height) of the first cylinder becomes large, so that correspondingly all of a cylinder material and the roller of the first rotary compression element, an eccentric portion, etc. have had to be replaced. Furthermore, as the thickness (or height) of the cylinder increases, the thickness (or height) of a rotary compression mechanism also increases, so that overall size of the relevant multi-stage compression type rotary compressor becomes larger, thus bringing about a problem of a difficulty in miniaturization of the compressor.
It is to be noted that the vane attached to such a multi-stage compression type rotary compressor is inserted movably in a groove formed in a radial direction of the cylinder. Such a vane is pressed against the roller to divide an inside of the cylinder into a low-pressure chamber side and a high-pressure chamber side in such a configuration that on a rear side of the vane a spring is provided to urge this vane on a roller side and also in the groove a back pressure chamber is provided which communicates with the high-pressure chamber of the cylinder to urge this vane on the roller side.
It is to be noted that in an internal medium-pressure type rotary compressor a pressure is higher in the cylinder of the second rotary compression element than in the sealed vessel, so that a pressure on a refrigerant discharge side of the second rotary compression element is applied to the back pressure chamber which urges the vane of this second rotary compression element.
If, for example, carbon dioxide (CO2) having a large difference between high and low pressures is used as the refrigerant, however, as shown in FIG. 5, a discharged refrigerant pressure reaches 12 MPaG in the second rotary compression element where it has the high pressure (HP). Accordingly, when a pressure on the refrigerant discharge side of the second rotary compression element is applied to the back pressure chamber, a pressure to press the vane against the roller becomes higher than necessary to thereby apply a large load on a portion where a tip of the vane slides along an outer periphery of the roller, thus bringing about a problem that the vane and the roller may be worn heavily or, in the worst case, be damaged.
Furthermore, a discharge-noise silencer chamber of each of the first and second rotary compression elements is provided with a discharge valve to prevent back-flow of the refrigerant when it is discharged into the discharge-noise silencer chamber, using which discharge valve the discharge port can be opened and closed when necessary.
It is to be noted that if, for example, carbon dioxide (CO2) having a large difference between high and low pressures is used as the refrigerant, as shown in FIG. 5, the discharged refrigerant pressure reaches 12 MPaG at the second rotary compression element where it has the high pressure (HP) and, on the other hand, becomes 8 MPaG (medium pressure: MP) at the first rotary compression element which is a lower-stage side at an outside air temperature of 15° C. (where the suction pressure LP of the first rotary compression element is 4 MPaG). As a result, a differential pressure at the first stage (difference between the suction pressure LP of the first rotary compression element and the discharge pressure MP of the first rotary compression element) becomes a large value of 4 MPaG. Moreover, with an increasing temperature of an outside air, the discharge pressure MP of the first rotary compression element increases rapidly, so that the first-stage differential pressure (difference between the suction pressure LP of the first rotary compression element and the discharge pressure MP of the first rotary compression element) increases further.
When the first-stage differential pressure increases in such a manner, a pressure difference between an inside and an outside of the discharge valve which opens and closes the discharge port of the first rotary compression element becomes excess, thus bringing about a problem of deterioration in durability and reliability such as damages of the discharge valve.
If the outside air temperature drops to reduce an evaporation temperature of the refrigerant, the discharge pressure MP of the first rotary compression element decreases, so that the second-stage differential pressure (difference between the suction pressure MP of the second rotary compression element and the discharge pressure HP of the second rotary compression element) increases further.
When the second-stage differential pressure increases in such a manner, a pressure difference between an inside and an outside of the discharge valve of the second rotary compression element becomes excess, thus bringing about a problem that the discharge valve etc. of the second rotary compression element may be damaged by this pressure difference.
Furthermore, the vane used in the rotary compressor is inserted movably in a guide groove provided in a radial direction of the cylinder. This vane, however, needs to be pressed toward the roller side always, so that conventionally, in configuration, the vane has been urged on the roller side not only by a spring but also by a back pressure applied to a back pressure chamber formed in the cylinder beforehand, thus complicating a construction.
Especially at the second rotary compression element of such an internal medium-pressure, multi-stage compression type rotary compressor, a pressure in the cylinder is higher than the medium pressure in the sealed vessel, thus bringing about a problem that a communication path needs to be formed through which a high back pressure is applied to the back pressure chamber.
Furthermore, in a refrigerant circuit using such a multi-stage compression type rotary compressor, an evaporator is liable to be frosted and so needs to be defrosted; however, if, to defrost this evaporator, a high-temperature refrigerant discharged from the second rotary compression element is supplied to the evaporator without being decompressed at a decompression device (in both cases of being directly supplied to the evaporator and being supplied thereto only by being passed through the decompression device but not being decompressed therethrough), the suction pressure of the first rotary compression element rises to thereby increase the discharge pressure (medium pressure) of the first rotary compression element. Thus, when this refrigerant is discharged through the second rotary compression element, it is not decompressed, so that the discharge pressure of the second rotary compression element becomes almost the same as the suction pressure of the first rotary compression element, thus bringing about a problem that a pressure level relationship may be reversed when the refrigerant is discharged from or sucked into the second rotary compression element.
This reversion in pressure level relationship during discharge and suction at the second rotary compression element can be avoided by providing such a refrigerator circuit as to supply the evaporator with a refrigerant discharged from the first rotary compression element without decompressing it so that the evaporator can be defrosted by supplying, using this refrigerant circuit, it with also the refrigerant discharged from the rotary compression element.
In this case, however, a discharge side of the first rotary compression element and that of the second rotary compression element communicate to each other in construction, so that a same pressure appears on the suction side and the discharge side of the second rotary compression element, thus bringing about a problem of unstable operation of the second rotary compression element such as breakaway of the vane from the second rotary compression element.